This invention relates to a metal nitride heat source and to improved methods for making the heat source. The methods and heat sources of this invention are particularly suitable for use in a smoking article, such as that described in copending U.S. Pat. application Ser. No. 223,153, filed on Jul. 22, 1988 (PM-1322) and commonly assigned herewith. The heat sources have low ignition and high combustion temperatures and generate sufficient heat to release a flavored aerosol from a flavor bed for inhalation by the smoker. Upon combustion, the heat sources of this invention produce substantially no carbon monoxide or nitrogen oxides.
According to the methods of this invention, the metal species is mixed with a carbon source, heated and converted to metal nitride by contacting the mixture with a nitriding material. In a preferred embodiment, the metal species/carbon source mixture is pre-formed into a desired shape and converted to metal nitride in situ, without substantially altering the shape of the mixture.
There have been previous attempts to provide a heat source for a smoking article. While providing a heat source, these attempts have not produced a heat source having all of the advantages of the present invention.
For example, Siegel U.S. Pat. No. 2,907,686 discloses a charcoal rod coated with a concentrated sugar solution which forms an impervious layer during burning. It was thought that this layer would contain gases formed during smoking and concentrate the heat thus formed.
Ellis et al. U.S. Pat. No. 3,258,015 and Ellis et al. U.S. Pat. No. 3,356,094 disclose a smoking device comprising a nicotine source and a tobacco heat source.
Boyd et al. U.S. Pat. No. 3,943,941 discloses a tobacco substitute which consists of a fuel and at least one volatile substance impregnating the fuel. The fuel consists essentially of combustible, flexible and self-coherent fibers made of a carbonaceous materials containing at least 80% carbon by weight. The carbon is the product of the controlled pyrolysis of a cellulose-based fiber containing only carbon, hydrogen and oxygen.
Bolt et al. U.S. Pat. No. 4,340,072 discloses an annular fuel rod extruded or molded from tobacco, a tobacco substitute, a mixture of tobacco substitute and carbon, other combustible materials such as wood pulp, straw and heat-treated cellulose or a sodium carboxymethylcellulose (SCMC) and carbon mixture.
Shelar et al. U.S. Pat. No. 4,708,151 discloses a pipe with replaceable cartridge having a carbonaceous fuel source. The fuel source comprises at least 60-70% carbon, and most preferably 80% or more carbon, and is made by pyrolysis or carbonization of cellulosic materials such as wood, cotton, rayon, tobacco, coconut, paper and the like.
Banerjee et al. U.S. Pat. No. 4,714,082 discloses a combustible fuel element having a density greater than 0.5 g/cc. The fuel element consists of comminuted or reconstituted tobacco and/or a tobacco substitute, and preferably contains 20%-40% by weight of carbon.
Published European patent application 0 117 355 by Hearn et al. discloses a carbon heat source formed from pyrolized tobacco or other carbonaceous material such as peanut shells, coffee bean shells, paper, cardboard, bamboo, or oak leaves.
Published European patent application 0 236 992 by Farrier et al. discloses a carbon fuel element and process for producing the carbon fuel element. The carbon fuel element contains carbon powder, a binder and other additional ingredients, and consists of between 60 and 70% by weight of carbon.
Published European patent application 0 245 732 by White et al. discloses a dual burn rate carbonaceous fuel element which utilizes a fast burning segment and a slow burning segment containing carbon materials of varying density.
These heat sources are deficient because they provide unsatisfactory heat transfer to the flavor bed, resulting in an unsatisfactory smoking article, i.e., one which fails to simulate the flavor, feel and number of puffs of a conventional cigarette.
Copending U.S. Pat. application Ser. No. 223,232, filed on Jul. 22, 1988 (PM-1319) and commonly assigned herewith, solved this problem by providing a carbonaceous heat source formed from charcoal that maximizes heat transfer to the flavor bed, releasing a flavored aerosol from the flavor bed for inhalation by the smoker, while minimizing the amount of carbon monoxide produced.
However, all conventional carbonaceous heat sources liberate some amount of carbon monoxide gas upon ignition. Moreover, the carbon contained in these heat sources has a relatively high ignition temperature, making ignition of conventional carbonaceous heat sources difficult under normal lighting conditions for a conventional cigarette.
Attempts have been made to produce non-combustible heat sources for smoking articles in which heat is generated electrically. E.g., Burruss, Jr., U.S. Pat. No. 4,303,083, Burruss U.S. Pat. No. 4,141,369, Gilbert U.S. Pat. No. 3,200,819, McCormick U.S. Pat. No. 2,104,266 and Wyss et al. U.S. Pat. No. 1,771,366. These devices are impractical and none has met with any commercial success.
Attempts have been made to produce a combustible, non-carbonaceous heat source. Copending U.S. Pat. application Ser. No. 281,496, filed on Dec. 12, 1988 (PM-1326) and commonly assigned herewith relates the use of a metal carbide heat source. Although combustion of the metal carbide heat source yields up to tenfold less carbon monoxide than combustion of conventional carbonaceous heat sources, some carbon monoxide is still produced.
Attempts have been made to produce pyrophoric materials comprising metal aluminides for use as a decoy for heat-seeking missiles. E.g., Baldi, U.S. Pat. No. 4,799,979. These devices, however, combust too rapidly and produce too intense a heat to be used as a heat source in a smoking article.
Methods of producing iron nitride by converting iron oxide to iron nitride are known. These methods generally involve treating metallic iron with a hydrogen/ammonia/molecular nitrogen mixture. E.g., K.H. Jack, Proceedings of the Royal Society, A. 195 pp. 34-40 (1948); K.H. Jack, Acta Crystallographica, 5, pp. 404-411 (1952); K.H. Jack, Proceedings of the Royal Society, A, 208, pp. 200-215 (1952); Knauff U.S. Pat. No. 3,681,018. These methods produced the iron nitride by a series of non-continuous steps, rendering these methods unsuitable for the large scale production of iron nitride. Moreover, the iron nitride produced by these methods possesses high thermal stability and low chemical reactivity and is, therefore, difficult to ignite, rendering it unsuitable for use as a heat source.
A further shortcoming of known methods of preparing iron nitride is that iron nitride is produced only in particulate form and must be formed into a shape suitable for use as a heat source. Iron nitrides by nature are brittle, intractable materials, which, once formed, are difficult and expensive to form into a suitable shape.
It would be desirable to provide a heat source that liberates virtually no carbon monoxide or nitrogen oxides upon combustion.
It would also be desirable to provide a heat source that has a low temperature of ignition to allow for easy lighting under conditions typical for a conventional cigarette, while at the same time having a combustion temperature high enough to provide sufficient heat to release flavors from a flavor bed.
It would further be desirable to provide a heat source that does not self-extinguish prematurely.
It would also be desirable to provide a heat source which is stable at ambient temperature and humidity.
It would be desirable to provide an inexpensive method of producing metal nitride which allows for control of end product distribution.
It would further be desirable to provide a method of producing metal nitride in which the starting materials are pre-formed into a desired shape and converted in situ to metal nitride.